Not applicable.
Various applications require the use of magnets designed to create a particular magnetic field, both in terms of the field pattern and strength. One such application is a proximity sensor for detecting a passing ferromagnetic article, such as a gear tooth. Conventional proximity sensors include a magnet and an integrated circuit having a Hall device. The integrated circuit is positioned in the magnetic field created by the magnet. In use, the Hall device generates an electrical signal related to the strength of the magnetic field normal to the plane of the Hall device. Thus, as a ferromagnetic article moves relative to the Hall device and the strength of the magnetic field changes, the electrical signal generated by the Hall device changes.
One way of tailoring the pattern and strength of the magnetic field provided by a magnet structure in order to provide suitable peak and open circuit characteristics is to use a magnetically permeable concentrator. The concentrator is positioned in close proximity to the magnet and effectively concentrates the magnetic field as a function of its material, geometry, and spatial relationship to the magnet.
In one such magnet structure, a magnetically concentrating steel plate is bonded between two semicircular pieces of sintered Samarium Cobalt (SmCo) magnetic material. The resulting structure may then be machined to provide a desired form factor for use in a particular application. For example, some conventional proximity sensor packages require the magnet structure to have a truncated semicircular cross-section.
The above-described magnet structure suffers certain drawbacks. In particular, costly bonding and aligning processes are required to assemble the magnet and concentrator and, even then, tight position tolerances may be difficult to meet. Further, over time, the concentrator may move relative to the magnet, thereby adversely impacting the device performance. Additionally, there is no easy way to xe2x80x9cfine tunexe2x80x9d the resulting magnetic field without experimenting with various concentrator materials, geometries, and placement relative to the magnet.
The invention relates to a magnet structure including a magnetic element and an integral magnetically permeable concentrator and methods for making the same. The integral formation of the concentrator and the magnetic element results in a unitary magnet structure in which the concentrator and magnetic element cannot be separated without destroying the structure. The concentrator and magnetic element are held together either by an interference fit and/or by a chemical bond. At least one of the components is formed by a molding process, with the other component placed in (and in one embodiment previously formed in) the mold.
In one embodiment, the concentrator has a substantially planar base with a post extending normal to the base and the magnetic element is provided in the form of a ring magnet having an outer diameter and an inner diameter defining a central aperture. The concentrator and ring magnet are formed such that the concentrator post extends into the central aperture of the ring magnet and at least a portion of the concentrator base is disposed adjacent to a surface of the ring magnet.
The concentrator is comprised of a plastic loaded with one or more magnetically permeable materials. Suitable plastics include thermoplastics, such as polyamide, polyphenylene sulfide (PPS), and polyphthalamide (PPA), and thermosets, such as epoxy molding compounds. Suitable magnetically permeable materials include iron, stainless steel, ferrite, and iron oxide. The plastic is loaded with the magnetically permeable material to a predetermined percentage by weight as the function of the desired magnetic field pattern and strength characteristics. As one example, the concentrator is comprised of polyamide loaded with iron particles to approximately five percent by weight.
The magnetic element may be comprised of a conventional magnetic material, such as sintered Samarium Cobalt (SmCo). Alternatively, the magnetic element may be comprised of a plastic loaded with one or more magnetic materials. Suitable plastics are as noted above for the concentrator and suitable magnetic materials include magnetic ferrite and SmCo.
Insert molding techniques are described for providing the integral concentrator and magnetic element. In accordance with one such technique, one of the components of the magnet structure (i.e., either the magnetic element or the concentrator) is prefabricated and placed in a cavity of a mold and a plastic compound is injected into the mold to contact at least a portion of the prefabricated component. The plastic compound is loaded with either magnetically permeable material or magnetic material, depending on which component is being formed. For example, in one embodiment, a sintered SmCo ring magnet is placed into a cavity of a mold and a magnetically permeable plastic compound is injected into the mold so as to contact one surface of the ring magnet and extend into the central aperture of the ring magnet, thereby forming the concentrator.
In accordance with an alternate insert molding technique suitable for fabricating a device in which the magnet is formed from a magnetically loaded plastic, a first plastic compound is injected into a mold cavity to form one of the components of the device (i.e., either a magnetic plastic compound is injected to form the magnet or a magnetically permeable plastic compound is injected to form the concentrator). Thereafter, the mold parts are moved relative to one another so as to form a further mold cavity and a second plastic compound is injected into this mold cavity to contact at least a portion of the previously formed component and form the other component of the structure.
Also described is a magnet structure including a ring magnet and a steel rod concentrator positioned in the central aperture of the ring magnet. In one embodiment, the steel rod is pre-cut to a length substantially equal to the height of the magnet and is held in place in the aperture of the ring magnet by a mechanical bonding process, such as with the use of an adhesive.